Tài liệu Anatomy of a Robot P2 pdf

PPP REPORT (WEEKLY) For the project, the PM should submit a Progress, Problems, and Plans (PPP) weekly report for several reasons. First, it’s a good record of your project. Second, you should at least have advisors who can look over the report and make suggestions. Last of all, it will light a fire under you since it’s embarrassing to file an empty report even if you’re the only person who reads it. You should observe a basic rule: Confess early and confess often. It’s much better to air problems early than to keep them secret and let them fester. They almost never get better left alone. Good management should reward PMs who turn themselves in because they have significant problems inside their project. That way adjustments and correc- tions can be made in a timely manner. Problems seldom get better when aged. The weekly PPP report you write should have the format shown here: PPP Report Project Report: 8/15/02 — Project XYZ Progress (The most important things that happened during the week) ■ ____________________________________ ■ ____________________________________ Problems (The most important problems that exist) ■ _____________________________________ ■ _____________________________________ Plans (The main short-term plans to be executed soon) ■ _____________________________________ ■ _____________________________________ PROJECT REVIEWS (SCHEDULED) The PM should schedule regular project reviews with senior advisors and colleagues. Some reviews are called for in the project schedule and checklist. Other reviews can be scheduled for corrective action, discovery, and so on. 16 CHAPTER ONE 01_200256_CH01/Bergren 4/17/03 11:23 AM Page 16 PM’S PROJECT CHECKLIST A PM should fill out the following checklist during the project. This is the best way to keep track of the requirements that must be met during the project. The manager to which you report will also be following this checklist. It’s your best tool to ensure that your needs will be met so you can execute the project cleanly. Robot’s name: _________________________________________________________ Manager: _____________________________________________________________ Start date: ___________ Targeted deliverable: ____________________________________________________ Dates: Project proposal approval: ______ Project plan approval: ______ Project resources assigned: ______ Spec reviewed: ______ HLD reviewed: ______ PPP reports (enumerate dates): ______ ______ ______ ______ ______ ______ Design reviews (as scheduled): ______ ______ ______ ______ ______ ______ Acceptance test completion: ______ Project completion: ______ Conclusion In summary, don’t overlook the fact that a project to build a robot must be properly man- aged like any other. Project management is an art and a field of study in its own right. PROJECT MANAGEMENT 17 01_200256_CH01/Bergren 4/17/03 11:23 AM Page 17 This page intentionally left blank. CONTROL SYSTEMS This chapter covers the most complex topics in the book. Control systems can be very ornate and difficult to build. They can be built using computers, linear electronics, mechanical parts, biological parts, or just spit and sticks! But underlying all control sys- tems is the queen of the sciences — mathematics. Given an understanding of the math, we can tame any of these types of control systems. In the final analysis, they all behave the same way, following the same math. It would be heresy to some to suggest that control systems can be tamed with an under- standing of just a few equations, but the fact is, the basic mathematical concepts of con- trol systems can be greatly simplified and made accessible. If you learn the basics, you can probably extrapolate to other cases using your instinct. That’s our goal in this chapter. Control systems are everywhere and they come in all shapes and sizes: ■ The average car has 35 computers in it now, running the engine, the brakes, the radio, the radar, and so on. ■ You are a control system of sorts. I can surely rely upon you to turn the page when my words run off this page to the next. You are very predictable that way and you fol- low the western standard of page turning, as does every school kid in the country. 2 19 02_200256_CH02/Bergren 4/17/03 11:23 AM Page 19 Copyright 2003 by The McGraw-Hill Companies, Inc. Click Here for Terms of Use. ■ Every toilet has a control mechanism for refilling the tank with the appropriate amount of water, and reliability is paramount. ■ The average toaster is great at browning bread in a repeatable manner. ■ You can probably walk through a completely dark room, touch a few well-known milestones, reach out with your hand, and find the light switch almost every time. We all take the existence of such control systems for granted. Let’s assume we’ve already built a large, strong robot body with the power, agility, strength, speed, and dex- terity we believe it needs. Now comes the hard part. Here’s a dream list of intangibles that might be really nice to have in the robot: ■ Intelligence ■ Wisdom ■ Compassion ■ Love ■ Perception ■ Communication skills That’s a long list, with many critical characteristics (that a good “person” should have) left off. How many of these things should we try to cram into the robot? Carl Sagan, the noted astronomer and author, once commented on the intellectual horsepower inherent in the control system of an interplanetary probe. He said the probe’s computer was roughly the intellectual equal of a cricket. To tell the truth, I think he sold crickets short (see Figure 2-1). So here’s a word of caution. If you hope to build a machine with wisdom and com- passion, you have a huge, impossible task before you. Here are some of the profound problems you’ll have to wrestle with. Forgive me for not explaining myself with all of these statements. I’d encourage you to consider each for yourself and delve into the rea- sons for these problems and their implications. 20 CHAPTER TWO FIGURE 2-1 Crickets are “smarter” than many computers. 02_200256_CH02/Bergren 4/17/03 11:23 AM Page 20 ■ The truth is, the human brain is capable of massive calculations, far more than the average huge computer. If you doubt this, consider the game of chess, in which humans have been beating computers for years. Computers designed for chess are only now catching up. But remember, chess is a game that a computer can at least digest easily, so the designers can optimize the computations. Most of life is much more complex than chess. ■ At the risk of throwing cold water on the dreams of creative young scientists, most acts of human interaction will probably never even be defined, much less equaled by machine. Wisdom, love, and compassion spring to mind. ■ The human mind has profound defects, defects that are manifest in the daily news broadcast. One could argue from an evolutionary standpoint that human defects such as those engendering greed and war are inevitable. Further, it could be argued these defects still benefit the human species and help to propagate it. It might be controversial to say so, but if we were to breed such traits out of humans, the insects would probably supplant us sooner than we might expect. As a side exer- cise, I ask you this. If you could press a button and make aggression, greed, envy, and other such vices instantly disappear from the human race, would you really press the button? If you could choose such traits for your robot, would you build them in? ■ Humans cannot know their own minds, much less duplicate them perfectly. It won’t stop us from trying though. ■ As a counterargument to my previous assertion, it must be stated that humans are having an increasingly difficult time distinguishing between human and computer “personalities.” Alan M. Turing, the British mathematician famous for his code-breaking work in World War II, proposed a simple experiment that has turned into a periodic contest. The experiment, known as the Turing Test, challenges a human interrogator to hold a conversation with two unseen enti- ties, one a computer and one a human. The interrogator must discover which is which. Winners are awarded the Loebner Prize. Visit the Loebner Prize web site for some interesting discussion and surprising results (www.loebner.net/Prizef/ loebner-prize.html). More on Turing can be found at http://cogsci.ucsd.edu/ ϳasaygin/tt/ttest.html#intro. ■ As another example of problems that cannot, and perhaps should not, be solved, consider whether your robot should be male, female, or genderless. We leave this exercise to the student body and recommend the debate be taken outside the classroom. A variant of the Turing Test, by the way, asks the interrogator to differentiate between a man and a woman. What questions would you ask? ■ Humans cannot communicate with each other perfectly. A person can only attempt to utter the right words that will instill the proper notion of his or her idea into CONTROL SYSTEMS 21 02_200256_CH02/Bergren 4/17/03 11:23 AM Page 21 another person’s mind. To communicate verbally, we form our thoughts, utter them, watch the reaction in the other person, and alter our statements based on his or her reaction. All these actions cannot be perfectly executed and always have unintended results. On this point, read Ronald David Laing’s book The Politics of Experience. A city with a large convention center suffered a flood inundating the first floor of the center. When firemen showed up at the convention center during the flood, they were amazed to hear rushing water every 45 seconds. Water gushed down the escalator from the second floor, stopped, and then repeated over and over. It turns out somebody had designed a smart feature into the elevator system. Since there were only two floors, why even bother putting in floor buttons? Just sense the motion of people coming into the elevator, and take them to the other floor. So the elevator was patiently going to the ground floor, opening up to allow the floodwater to come in, and bringing it to the sec- ond floor. Sensing a great deal of traffic, the elevator returned to the ground floor for more “people.” All the while, the control system was perfectly content with its actions. So my advice about the control system is this. Keep it simple, unless you’re just experimenting and fully prepared to fail. Let’s take a step back and look at your original goals. If you’ve written specifications for the robot (and kept them simple), you have a limited list of tasks that the robot must perform. All you have to do is build a robot that can execute the tasks on its plate. Where do we start designing a robot so it can do such things? For starters, we can look to nature for analogous designs. Nature abounds with control systems worthy of emulation. However, our thoughts are commonly rife with anthropomorphic visions of robots. The first image that springs to mind is of a robot with a head, two eyes, two ears, a mouth, two arms, and a torso. Are we being led astray by our own instincts? Distributed Control Systems Although many arguments have been made for the existence of a distributed intelligence within the human body, clearly a central control system exists: the brain. Is a central con- trol system what we really want? This is worth considering before choosing an architecture. Consider a school of herring. They swim in giant schools, flashing silvery in the deep blue ocean light. See http://www.actwin.com/fish/marine-pics/anchovie.mpg. As some tuna come in to attack, the school instantly swerves, divides, and coalesces as if by magic. It’s a viable survival tactic for the herring. How do they pull off such a feat? Well, 22 CHAPTER TWO 02_200256_CH02/Bergren 4/17/03 11:23 AM Page 22 each individual herring simply watches his four immediate neighbors and reacts to their position, speed, and movement. The net result, observed at the school level, is dramatic and effective. Thousands of tiny brains act almost as one, and the tuna are partially frus- trated. With luck, they go off to bother the shrimp. The herring school is using a “distributed” control system. The school is governed by the collective will and common actions of the individual fish. Consider some of the advantages of a distributed control system: ■ Cheapness Individual control systems elements are simpler and cheaper. In this example, we’d only have to design something simple like a herring and then repli- cate it thousands of times (gaining economies of scale). ■ Reliable If the system is designed to survive the failure of portions of the sys- tem, a few failures will not bring it down. Surely, not all the herring escape the tuna. The school simply changes shape to heal up the hole where the eaten her- ring once was, and life goes on. A distributed control system does have some disadvantages: ■ Communication Sometimes it’s hard to communicate everything between indi- vidual control elements. A herring at the far side of the school doesn’t know a tuna is coming until his neighbor signals such. The panic signal spreads through the school like a wave, but it might be too late. This form of knowledge truly is power, and a matter of life and death. ■ Horsepower The individual elements within a distributed control system gen- erally are not powerful in and of themselves. Although the collective herring school solves the tuna problem as well as any human or computer might, the indi- vidual herring could not match a human at math or reasoning. Distributed control systems are often designed to solve specific problems and are not as good at field- ing general-purpose problems. If you use a distributed control system, be very careful that you know all the problems that it must face. If the specifications change, your design might flounder! If you’d like to explore a distributed model for robot control, here are some URLs with source software and links. Just beware; you could easily spend weeks playing with these models: ■ http://www.red3d.com/cwr/boids/ The following URLs consider general-purpose distributed control systems: ■ www-db.stanford.edu/ϳburback/dadl/ ■ www-db.stanford.edu/ϳburback/dadl/node87.html CONTROL SYSTEMS 23 02_200256_CH02/Bergren 4/17/03 11:23 AM Page 23 One of the purposes of this book is to point out fields of endeavor that might lead you to a life-long career choice. If, for some odd reason, you’re hooked on herring, go to Iceland (http://siglo.is/herring/en/silver.shtml)! Central Control Systems Let’s take a look at centralized control systems. Certainly, an understanding of a single control system is vital for an understanding of a distributed control system. I’m going to leave it as an exercise to extrapolate these teachings to any work done on a distrib- uted control system. Most control systems are built around the same basic control structures. We’ll look at a few different structures, but the point is their behavior can be described by the same math. We can discover for ourselves the sorts of characteristics that these control sys- tems have by observing a readily available control system. The control system I’ve cho- sen to demonstrate is, right now, at the tip of your finger. We are shortly going to do some experiments while you are reading. Open-Loop Control Most robot control systems have some sort of input signal and output signal. In between, the control system responds to the input signal and changes the output signal accord- ingly. The following is a simple diagram showing an open-loop control system (see Figure 2-2). The input signal is generally a low-level control signal. Two examples of an input sig- nal might be the signal from the power button on a TV remote or the linear voltage from a rotating dimmer switch. Generally, in a control system, the actuator amplifies and transforms the input signal. When a person presses the power button on the TV remote, the remote generates an infrared signal that the TV interprets to close a relay and give 24 CHAPTER TWO FIGURE 2-2 An open-loop control system Actuator Input Signal Output Signal 02_200256_CH02/Bergren 4/17/03 11:23 AM Page 24 power to the TV circuits. Actually, two open-loop control systems are at work. They are concatenated and operate as a single open-loop control system (see Figure 2-3). In open-loop control systems, the information tends to flow only one way. For exam- ple, the control system inside the remote never finds out if the TV goes on or not. Furthermore, the power button on the remote never indicates if the infrared beam was sent out or not. If your finger is over the optical opening, nothing happens at all and the remote never knows the TV has not gone on. Let’s run an experiment illustrating an open-loop control system within your body. Glance over to your right and locate an object in the room. Remember where it is and then look back here to the book. Now close your eyes, point to the object, trying to put your finger right on the object in your field of vision. Open your eyes, and see how close you came (see Figure 2-4). You’ll notice that you never really get it right with your eyes closed. When you open your eyes, you can see your finger is a little off. The error will never go away and is called the steady state error. It’s an error that will persist long after the control system has settled on the final output and will make no further corrections. We’ll see steady state error as a term in the equations that we develop later. All control systems have this error. It’s an important parameter because when you are designing a control system, you must keep the steady state error below acceptable bounds. You can perform another experiment if you have a dimmer in your home. Wait until dark and turn off the dimmer, making the room dark. Close your eyes and then turn on the dimmer to where you think the minimum acceptable reading light level is. CONTROL SYSTEMS 25 FIGURE 2-3 Concatenated open-loop control systems Power Butt Infrared Signal TV Power to TV TV Remote FIGURE 2-4 The open-loop control error is large with eyes closed. 02_200256_CH02/Bergren 4/17/03 11:23 AM Page 25 [...]... that a moving mass might not just be moving linearly It might also be rotating As such, you can model the energy of both motions separately You can use the center of gravity of the mass and see how fast that is moving linearly Then you can add the energy of rotation about that center of mass (as you find it) Mass at heights (potential energy) When a mass is at a height, the potential energy it has... steady state error coefficient to estimate what that error will be in advance and design the robot to allow for an error of that size If the system has too much steady state error, consider revising the actuator gain to correct it CONTROL SYSTEMS 29 I We might be led to believe that making the actuator gain as large as possible is desireable Just be aware that increasing the gain of the actuator adds... providing extra control elements (in this case, vision) STEADY STATE ERROR Now that we’ve identified a parameter of interest, let’s look at the math We can assign arbitrary variables to represent the signals and control system elements that we have been talking about (see Figure 2-7) Looking at the circular arithmetic element (subtraction), b a d The actuator is said to have a gain of C This gain can be immense... start with physics, calculus, Laplace transforms, and algebra to arrive at usable results Once we have that math in front of us, we will explore the tools it affords us First, we need a way to look at the parts of the robot and assign numbers to the movements we observe This can be done in a couple of ways: I Energy evaluation One way to analyze dynamic movement is by looking at everything in terms of. .. need to analyze and manipulate the performance of the robot, we’re going to pick a mathematical model for the robot and derive some of the equations We’re going to skip the easier models of robot behavior and go straight to a slightly more complex case We are going to use math and physics that might be beyond the casual reader’s abilities, but we will return to a usable, intuitive model of what’s going... the actuator adds expense and will adversely affect the dynamic (nonsteady state) behavior of the control system as we will see later In the worst case, a large actuator gain can make the system unstable and lead to failures Whenever altering the gain, remember to reevaluate and retest the dynamic performance of the control system Realize that these equations model a general-purpose closed-loop control... ϭ m ϫ a for the mass F ϭ B ϫ v for the friction, so B ϭ m ϫ a/ v This technique works for rotational, linear, or spring-type movements So now we have to pick a mechanical model of the robot in order to make a mathematical model for it We will pick an arbitrary model that will probably be different than our robot s actual mechanics However, once we learn how to analyze and manipulate this arbitrary model,... is meant to control the robot s position, then the variables a, b, and d are measured in distance If the control system is meant to control the robot s speed, the variables are measured in speed If the control system is meant to control the robot s acceleration, the variables are measured in acceleration The fundamentals of the math are still the same; only the units change We can use the equations... control any of the aforementioned systems without further investigation We leave it up to the reader to investigate the mathematics of calculus that hold that acceleration is the derivative of velocity, and velocity is the derivative of position Suffice it to say that positive acceleration builds up speed, negative acceleration (braking or accelerating in reverse) decreases speed, positive speed accumulates... immense and the system will still work As an example, if a very tiny positive signal takes place at b, then signal d can be extremely large and positive Similarly, if a very tiny negative signal is issued at FIGURE 2-6 The closed-loop control error is smaller with eyes open 28 CHAPTER TWO Feedback _ Input Signal a + b Actuator Gain = C Output Signal d FIGURE 2-7 A closed-loop system with an actuator and . is paramount. ■ The average toaster is great at browning bread in a repeatable manner. ■ You can probably walk through a completely dark room, touch a few. be aware that increasing the gain of the actuator adds expense and will adversely affect the dynamic (nonsteady state) behavior of the control system as